Honda has been accused of union-busting after Unite, Britain's largest union, uncovered company documents referring to minimising its influence at the car company's UK operations.

General secretary Len McCluskey is writing to shareholders to warn that the Japanese company could derecognise the union, barring it from Honda premises including the Swindon factory where 3,000 people work. He acted after Unite's senior shop steward at Honda was suspended after printing off two slides from the Honda UK internal website, headed "mid-term review", which use the phrases "minimise influence of trade union" and "accelerate independence of ARC [Associate Representative Council]", referring to the elected body Honda uses for pay bargaining but which is currently dominated by Unite representatives.

One slide, apparently covering a period from 2008 to 2011, includes the phrase "derecognition claim", though the company could not confirm if it refers to bid to derecognise Unite by a group of Honda employees this year.

In a draft letter to Honda's UK-based institutional shareholders, including JP Morgan and Lazard, McCluskey says the slides "evidenced a plan on the part of Honda management to move towards a reduction in the role of Unite, the logical outcome of which would be derecognition". Tony Murphy, Unite's national officer for the car industry, said: "They know that without the union in there they will be able to dictate terms and conditions."

A Honda spokeswoman said: "I cannot comment on whether they are Honda documents. That is part of an ongoing investigation." However, Jim D'Avila, a Unite official, said he had seen the slides on Honda's intranet. The slide that is understood to refer to Honda's strategy for the next four years also makes reference to "minimise influence of trade union", next to a box with the words "trade union Unite".

Honda has invested £1.4bn in Swindon, which started building engines in 1989 and cars three years later. This year the company set a UK production target of 135,000 vehicles.

Union derecognition is a tactic rarely used by employers in the UK, not least because it leaves them without a single point of contact to negotiate pay and conditions with hundreds or thousands of staff. Plymouth council derecognised Unison in August after it refused to sign a collective agreement on contractual changes, but re-recognised it last month after other unions refused to go ahead with the deal without Unison.